Electronically tuned self-starting polarization shaping mode locked fiber laser
Patent 7477665 Issued on January 13, 2009. Estimated Expiration Date: 
February 9, 2026. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.
February 9, 2026. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.Patent ReferencesPolarization mode dispersion compensation via an automatic tracking of a principal state of polarization H1926 Pulse-generating laser Multiple polarization combiner-splitter-isolator and method of manufacturing the same Patent #: 6782146 InventorAssigneeApplicationNo. 11351994 filed on 02/09/2006US Classes:372/6OPTICAL FIBER LASERExaminersPrimary: Harvey, MinsunAssistant: Stafford, Patrick Attorney, Agent or FirmInternational ClassesH01S 3/30H01S 3/10 DescriptionFIELD OF THE INVENTIONThe present invention relates generally to apparatuses and methods for providing short-pulsed mode-locked fiber laser. More particularly, this invention relates to new configurations and methods for providing a self-started polarization-shapingmode locked fiber lasers. BACKGROUND OF THE INVENTION Conventional technologies of generating short pulse mode-locked fiber laser are still confronted with technical difficulties and limitations. The pulse shapes of the short-pulse laser cannot be properly and conveniently controlled. Thedifficulty is even more pronounced when the pulse width is further reduced. Due to this difficulty in pulse shape control, conventional technologies are not able to provide an automatic controller to self-start a fiber laser system with an automaticpolarization-shaping mode locked option. There is an urgent demand to resolve these technical difficulties as the broader ranges of applications and usefulness of the short pulse mode-locked are demonstrated for measurement of ultra-fast phenomena,micro machining, and biomedical applications. An active pulse shaping mode locked fiber laser was disclosed by J. D. Kafka, T. Baer, and D. W. Hall in a paper entitled "Mode locked erbium doped fiber laser with soliton pulse shaping," Opt. Lett. 22, 1269-1271 (1989). Different from theactive pulse shaping mode locked fiber laser, intensity dependent polarization rotation or nonlinear polarization evaluation (NPE) has been identified as a fast response saturation absorber (SA) to achieve short pulse fiber laser as presented by C. J.Chen, P. K. Wai, in "Soliton fiber ring laser," Opt. Lett. 17, 417-419 (1992). However, D. U. Noske, N. Pandit, J. R. Taylor and K. Tamura, H. A. Haus, and E. I. Ippen have showed by their experimental results that longer pulse widths and come withunwanted sidebands that degraded the performance of the soliton fiber lasers. More details can be referred to D. U. Noske, N. Pandit, J. R. Taylor, "Subpico-second soliton pulse formation from self mode locked erbium fiber laser using intensitydependent polarization rotation," Electronics Letters 28, 2185 (1992) and K. Tamura, H. A. Haus, and E. I. Ippen, "Self starting additive pulse mode locked erbium fiber ring laser," Electonics Letters 28, 2226 (1992). To further reduce the pulse width,stretched pulse fiber laser were proposed using short length of fiber cavity and operating at positive dispersion region. A 77 fs pulse fiber laser has been demonstrated. These demonstrations were discussed in K. Tamura, et al., "77 fs pulse generationfrom a stretched pulse mode locked all fiber ring laser," Opt. Lett. 18, 1080 (1993) and Tamura, et al., Stretched pulse fiber laser, U.S. Pat. No. 5,513,194, 1996. However, they have not achieved transform-limitedly shaped pulse, because thespectrum is not symmetrically Gaussian/Soliton shape and time bandwidth product (TBP) is too large. It is still remained a challenge to obtain transform limited pulse. More specifically, in U.S. Pat. No. 5,513,194 Tamura et al. disclosed a fiber laser for producing high-energy ultra-short laser pulses, having a positive dispersion fiber segment and a negative-dispersion fiber segment joined in series with thepositive-dispersion fiber segment to form a laser cavity. With this configuration, soliton effects of laser pulse circulation in the cavity are suppressed and widths of laser pulses circulating in the cavity undergo large variations between a maximumlaser pulse width and a minimum laser pulse width during one round trip through the cavity. The fiber laser also provides means for mode-locking laser radiation in the laser cavity, means for providing laser radiation gain in the laser cavity, and meansfor extracting laser pulses from the laser cavity. Using selected positive- and negative-dispersion fiber segments, the laser cavity exhibits a net positive group velocity dispersion, and the ratio of the maximum laser pulse width to the minimum laserpulse width attained during one round trip through the cavity is greater than 5, and preferably greater than 10. The laser cavity may be configured with different cavity geometries and preferably the ring cavity to achieve unidirectional circulation oflaser pulses to produce laser pulses having a pulse width of less than 100 fs and a pulse-energy of at least 80 pJ. However, as that shown in FIG. 1, the waveform of the short pulse laser still present distorted pulses and the laser so generated is nota transform-limited shape and still have limited applications in telecommunications since such laser pulse is not able to overcome the problems of the non-linearity and dispersion effects of the laser pulses during the transmission. The distorted pulseshapes are caused by the unbalanced dispersion and the non-linearity of control for operating the laser at the positive net dispersion region. For these reasons, the laser disclosed by Tamura et al. cannot achieve a higher laser transmission efficiencyof the transformed-limited shape. Therefore, a need still exists in the art of fiber laser design and manufacture to provide a new and improved configuration and method to provide short pulse mode-locked fiber laser with better controllable pulse shapes such that the abovediscussed difficulty may be resolved. Furthermore, in order to provide reliably controllable fiber laser system that can be conveniently tuned and operated, it is further desirable to provide f electronically tunable fiber laser systems. Additionally,it is further desirable that the electronically tunable system can be self-starting with polarization shaping and mode-locked operational functions such that time savings can be achieved in starting and operating the laser system. SUMMARY OF THE PRESENT INVENTION It is therefore an aspect of the present invention to provide a method of using nonlinear polarization evolution (NPE) and dispersion managed fiber cavity to manipulate the pulse propagation in the cavity and balance the self phase modulation(SPM) and dispersion induced pulse broadening/compressing. This method of polarization pulse shaping generates transform-limited pulse shapes through combinational effects of fiber length, the non-linear effects and dispersion such that theabove-described difficulties encountered in the prior art can be resolved. This invention further provides a method to self-start a polarization shaping mode locked fiber laser by electronically tuning a polarization controller. The in line electronically driven polarization controllers may be alternately implementedwith either a liquid crystal (LC) based, squeezed fiber based, or any other types. These polarization controllers as implemented and the optical components included in the fiber laser systems of this invention can be in line type or free space types. Specifically, it is an aspect of this invention to provide a new short-pulse mode-locked fiber laser with a ring structure that includes a 980/1550 WDM (wavelength division multiplexing) coupler for introducing a laser projection to a gain mediumto amplify the pulse for transmitting in a cavity that includes fiber segments of positive and negative dispersions. The laser transmission then passes through a polarization sensitive isolator controlled by polarization controller to carry out a pulseshaping for generating extra-short pulse of laser conforming to the transform-limited pulse shapes. It is a further aspect of this invention that the polarization controller is connected to an electronically tunable driver to provide a self-startingpolarization shaping mode locked fiber laser system. Briefly, in a preferred embodiment, the present invention discloses a fiber laser cavity that includes a laser gain medium for receiving an optical input projection from a laser pump. The fiber laser cavity further includes a positive dispersionfiber segment and a negative dispersion fiber segment for generating a net negative dispersion for balancing a self-phase modulation (SPM) and a dispersion induced pulse broadening/compression in the fiber laser cavity for generating an output laser witha transform-limited pulse shape. The fiber laser cavity further includes a polarization sensitive isolator and a polarization controller for further shaping the output laser. The polarization controller further controlled and driven by anelectronically tunable self-starting pulse shaping driver tapping a small portion of an output laser of the fiber laser cavity to process and filtering the small portion of the output laser to drive the polarization controller. In a preferredembodiment, the gain medium includes an erbium doped fiber constitutes a positive dispersion fiber segment. In a preferred embodiment, the laser cavity is a ring cavity. The laser cavity further includes an output coupler for transmitting a portion ofa laser as the output laser from the fiber laser cavity. In another preferred embodiment, the laser cavity further includes a single mode fiber constituting a fiber segment of a negative dispersion connected to the gain medium. In a preferredembodiment, the gain medium further includes a Ytterbium doped fiber for amplifying and compacting a laser pulse. In a preferred embodiment, this invention further discloses a method for method for generating a pulse-shaped transform-limited output laser from a laser cavity that includes a laser gain medium. The method includes a step of forming the lasercavity by employing a positive dispersion fiber segment and a negative dispersion fiber segment for generating a net negative dispersion. The method further includes a step of projecting an input laser from a laser pump into said fiber laser cavity forbalancing a dispersion induced nonlinearity with a self-phase modulation (SPM) in said fiber laser cavity for generating an output laser with a transform-limited pulse shape. The method further includes a step of electronically tuning a polarization ofa laser transmission in the laser cavity by tapping a small portion of the output laser for processing, filtering and driving a polarization controller for enabling a self-starting polarization shaping and mode locking process. In another preferred embodiment, this invention further discloses a fiber laser cavity that includes a laser gain medium for receiving an optical input projection from a laser pump. The fiber laser cavity further includes a positive dispersionfiber segment and a negative dispersion fiber segment for generating a net negative dispersion for balancing a self-phase modulation (SPM) and a dispersion induced pulse broadening/compression in the fiber laser cavity for generating an output laser witha transform-limited pulse shape. The fiber laser cavity further includes a polarized insensitive isolator for receiving a collimated beam from a collimator coupled to a polarization controller for projecting an isolated beam to a beam splitter forgenerating a polarized transform-limited output laser. The fiber laser cavity further includes a polarization controller controlled and driven by an electronically tunable self-starting pulse shaping driver tapping a small portion of an output laser ofthe fiber laser cavity to process and filtering the small portion of the output laser to drive the polarization controller. These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill inthe art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the pulse shape generated by a prior art invention. FIG. 2 is functional block diagram for a short-pulse mode-locked fiber laser of this invention. FIG. 3A is a waveform for showing the output spectrum from a mode-locked fiber laser as shown in FIG. 1. FIG. 3B is a waveform for showing an autocorrelator trace for pulse width measurement (150 fs). FIG. 3C shows the pulse broadening/compression ratio as function of the peak power for a 1 m fiber with dispersion of 17 ps/nm/km and a pulse width of 200 fs. FIGS. 4A and 4B are waveforms for showing polarization changes as laser pulse transmitted over a laser cavity. FIG. 5 is a functional block diagram of another preferred embodiment of an all fiber-based short-pulse mode-locked laser of this invention. FIG. 6 is a functional block diagram of another preferred embodiment of a nonlinear polarization pulse shaping short-pulse mode-locked laser of this invention. FIG. 7A is a functional block diagram of another preferred embodiment of an alternate nonlinear polarization pulse shaping short-pulse mode-locked laser of this invention. FIG. 7B is a diagram for showing a type of polarization controller with a collimator. FIGS. 8 and 9 are two fiber systems provided with electronic driving system to enable the fiber laser to achieve electronically tuned self-starting polarization shaping mode locked functions. FIG. 10 is a function block diagram for showing the electronic driving system to drive and control the polarization controller of FIGS. 8 and 9. FIG. 11 is functional block diagram for showing a high power amplifier for providing a femtosecond laser pulses. DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 2 for a schematic diagram of a nonlinear polarization pulse-shaping mode locked fiber laser 100 of this invention. The fiber laser is a ring structure laser that includes a gain medium (EDF) 105, a polarization sensitiveisolator 135, polarization controllers 140-1 and 140-2, a 980/1550 WDM coupler 110, and an output coupler 130. Alternately, the gain medium 105 may also be implemented with a Yatterbium (Yb) doped fiber (YDF). One meter of Erbium doped fiber (EDF) 105was used in the fiber laser as a gain medium and is used to amplify and compress the pulse width. The fiber has a high doping concentration (80 dB/m at 1530 nm) with a dispersion of -55 ps/nm/km. A 980 nm high power pump laser diode 101 coupled througha wavelength division multiplexer 110 is used to pump the EDF 105 to amplify the pulses circulating in the cavity. The rest of the cavity comprising a single mode (SM) fiber (17 ps/nm/km) 115 having a length about three meters and an HI 1060 fiber 120commercially provided by Corning as standard fiber with dispersion 14 ps/nm/km at 1550 nm having a length of about 0.5 meter. The output fiber pigtail 125 includes a single mode (SM) fiber having a length of about one meter. A coupler 130, e.g., a10%/90% coupler, is coupled between the single mode (SM) fiber 115 and the output fiber to transmit a portion of the light, e.g., 90% of the light, out of the cavity. The coupling ratio can also be adjusted to obtain different power levels of output. The gain medium EDF 105 has a normal dispersion fiber (β''>0) and remainder portions of the fibers are negative dispersion fibers (β''<0), the whole cavity average dispersion is designed to operate at anomalous dispersion (β''<0)to achieve a stable transform-limited pulse. The fiber laser 100 of this invention is different from the conventional laser as that disclosed by C. J. Chen, P. K. Wai, in "Soliton fiber ring laser," Opt. Lett. 17, 417-419 (1992) and D. U. Noske, N. Pandit, J. R. Taylor, in "Subpicosecondsoliton pulse formation from self mode locked erbium fiber laser using intensity dependent polarization rotation," Electronics Letters 28, 2185 (1992). The fiber laser 100 of this invention generates the transform-limited short pulse mode locked fiberlaser by a combination of negative and positive dispersion fibers to manage the pulse propagation in the cavity and balance the self phase modulation (SPM) and dispersion to reduce the saturation effects in the amplification region. The erbium-dopedfiber (EDF) 105 is a positive dispersion fiber and the remaining portions of the fibers are negative dispersion fibers. The ratio of the positive to the negative dispersions in one of the preferred embodiment is approximately 2 to 5. When the nonlinearlength and dispersion length are comparable, e.g., within a ratio of 1-3, the shape of soliton, or other transform-limited pulse, maintains the pulse shape while in propagation through either transmission fiber or gain medium. The nonlinear length,i.e., Lnl=1/γP, where P is the peak power of the pulse and γ is nonlinear coefficient, and the dispersion length, i.e., Ld=T^2/|β''|, where T is the pulse width, provides the length scale over which the dispersive effects or nonlineareffects become important for pulse evolution along a fiber segment. When the fiber length is longer or comparable to both the dispersion length Ld and the nonlinear length Lnl, the dispersion and the nonliearity work together for pulse propagation alongthe fiber. Actual implementation of the laser configuration substantially similar to a system shown in FIG. 2 has been performed in the laboratory. Adjustments for tuning the two polarization controllers 140-1 and 140-2 are carried out by adjusting thecurrent to 250 mW to generate pulsed output. The mode locked pulses are self-started when the laser is turned on and can be maintained down to 100 mW. FIG. 3A shows its spectrum in linear scale with a bandwidth of 20 nm. It is very close to a solitonshape. FIG. 3B represents its pulse width measurement by using an autocorrelator (150 fs). The TBP is calculated to be 0.37, indicating a good soliton pulse. The repetition rate is about 50 MHz. Average power can be varied from 1 mW to 5 mW. FIG. 3Cshows a computational results of the pulse broadening-/compression ratio as function of the peak power for a 1 m fiber with dispersion of 17 ps/nm/km and a pulse width of 200 fs. A fiber exhibits a nonlinear birefringence that depends on the local intensities of the two orthogonally polarized field components. As a result, an elliptically polarized pulse will have two orthogonal components, i.e., x and y components. These two components experience different phase shifts, thus rotating the polarization ellipse. Since the phase shift is an intensity-dependent process, it rotates the polarization of a pulse at different amounts depending on the pulse's localintensity. FIGS. 4A and 4B show polarization's physical effect on a pulse. If the nonlinear effects are ignored and let FIG. 4A represent a uniformly polarized pulse launches into an isotropic optical fiber, a uniformly polarized output pulse isobtained as that depicted by FIG. 4B. Therefore, by launching the same pulse into the same fiber implemented with the effects of self phase modulation (SPM) and Cross phase modulation (XPM), an output similar to FIG. 4B can be generated. Examining FIG.4B, it is noted that the low intensity wings are not affected, yet, as the pulse's intensity increases, a rotation of the polarization ellipse is observed. Therefore, a nonlinear phase evolution (NPE) induced by the nonlinear phase change of self-phasemodulation (SPM) causes a polarization rotation, as the polarization is now pulse intensity dependent. Thus, the mode lock mechanism is caused by the SPM induced NPE. When the pulse passes through the polarization sensitive isolator that is controlledand adjusted by a polarization controller, only the highest intensity that lined up with the isolator will pass. The lower intensity part of the pulse is filtered out. Therefore, the pulse is well shaped and thus works as a saturation absorber (SA) toreduce the pulse width. The polarization controller 140 can be fiber based, or bulk optical quarter/half wave retarders, or a combination of both. The "polarization sensitive isolator and polarization controllers" works to select a polarization for thepulses, which have different polarization states in time domain. When the pulse circulates in the fiber laser cavity, the laser pulse experiences the self-phase modulation (SPM) induced pulse broadening effects in both negative anomalous single mode fibers and positive normal dispersion fiber regions due to ahigh peak power and short pulse width (0, in the EDF 105, because the peak power is very high (>200 W for a 200 fs pulse), the nonlinear length and the dispersion length arecomparable, i.e., ~1 m, in the EDF 105 segment. The pulse can be compressed by using the effects of both self phase modulation (SPM) and dispersion. Referring to FIG. 5 for a mode-locked fiber laser 100' formed with all fiber-based components. The fiber laser has a ring configuration receiving a laser input through a 980 or 1550 nm WDM 110. The all fiber-based laser 100' similar structureas that shown in FIG. 2 with an erbium doped fiber 105 as a gain medium to amplify and compress the pulse width. The all fiber-based laser 100' employs an in-line polarization controller 140-1' and 140-2' before and after an in-line polarizationsensitive isolator 135' that is implemented with single mode (SM) fiber pigtails. The in-line polarization sensitive control may be a product commercially provided by General Photonics, e.g., one of PolaRite family products. Similar to the laser shownin FIG. 2, the gain medium EDF 105 has a normal dispersion fiber (β''>0) and rest of the fibers are negative dispersion fibers (β''<0), the whole cavity average dispersion is designed to operate at anomalous dispersion (β''<0) toachieve a stable transform-limited pulse. All fiber solution as shown provides advantages of compactness and robustness thus more suitable for practical industrial applications. Referring to FIG. 6 for a preferred embodiment similar to that shown in FIG. 2 to achieve an all fiber solution for mode-locked fiber laser to generate an output polarized laser. Instead of an output coupler 130 as that implemented in FIGS. 2and 5, an in-line beam splitter 130' is used that has a single mode input fiber with a polarization maintaining (PM) output fiber 125'. The in-line isolator 135 can be either a polarization sensitive or a polarization insensitive isolator. The outputfrom the PM output fiber 125' provides seed pulses for further pulse amplification and second harmonic generation. Referring to FIG. 7A for another preferred embodiment similar to that shown in FIG. 2 to generate a polarized output by placing a polarization splitter 130'' between the collimators of the polarization controllers 140-1 and 140-2. Thisembodiment is configured with some free space components. FIG. 7B shows the inline polarization controller 140-1 is connected to a collimator 145 for collimating the light for projecting to the polarization sensitive isolator 135 and the polarizationbeam splitter 130''. This embodiment provides the advantageous features of generating polarized output and application of a fiber-based polarization controller. FIG. 8 an alternate embodiment of this invention similar to the fiber laser systems as shown in FIG. 2 and FIG. 6 to achieve an all fiber solution for mode-locked fiber laser to generate an output polarized laser. Instead of an output coupler130 as that implemented in FIGS. 2 and 5, an in-line beam splitter 130' is used that has a single mode input fiber with a polarization maintaining (PM) output fiber 125'. The in-line isolator 135 can be either a polarization sensitive or a polarizationinsensitive isolator. The output from the PM output fiber 125' provides seed pulses for further pulse amplification and second harmonic generation. The in-line polarization controller 140-2'' is now implemented as an electrically driven polarizationcontroller. Referring to FIG. 9 for another mode-locked fiber laser 100'' similar in structure and function as the laser fiber system of FIG. 5 formed with all fiber-based components. The fiber laser has a ring configuration receiving a laser input througha 980 or 1550 nm WDM 110. The all fiber-based laser 100'' similar structure as that shown in FIG. 2 with an erbium doped fiber 105 as a gain medium to amplify and compress the pulse width. The all fiber-based laser 100' employs an in-line polarizationcontroller 140-1' and 140-2''' before and after an in-line polarization sensitive isolator 135' that is implemented with single mode (SM) fiber pigtails. The in-line polarization sensitive control may be a product commercially provided by GeneralPhotonics, e.g., one of PolaRite family products. Similar to the laser shown in FIG. 2, the gain medium EDF 105 has a normal dispersion fiber (β''>0) and rest of the fibers are negative dispersion fibers (β''<0), the whole cavity averagedispersion is designed to operate at anomalous dispersion (β''<0) to achieve a stable transform-limited pulse. All fiber solution as shown provides advantages of compactness and robustness thus more suitable for practical industrialapplications. The polarization controller 140-2''' is driven by an electronic driving system as shown in FIG. 10 below. By connecting the polarization controller 140-2''' to the electronic driving system through connecting pins 155, the fiber lasersystem 100''' is enabled to achieve an electronically tuned self-starting polarization shaping mode-locked function. FIG. 10 is a functional block diagram to illustrate the electronic driving system connected from the controller electronics 150 via connector pins 155 to the in-line polarization controller 140-2''. In order to automatically start thepolarization shaping fiber laser and electronically drive the polarization controller 140-2'', a small percentage of signal, e.g.,1% is tapped out from the output coupler 130. A photo detector 160 detects the tapped signal and transmitting the detectedsignals to a processing electronics 165 and a filtering electronics 170. The processing electronics 160 and the filtering electronics 165 generate DC components of the signals and the 20-100 MHz band components that is the laser repetition rate. Theprocessing electronics 160 and the filtering electronics 165 further use the other signature frequencies such as relaxation frequency. The controller electronics 150 then correlates these components, e.g., the DC, 20-100 MHz, and/or relaxationfrequency, with the polarization state controlled by the polarization controller in the cavity, self-start can be achieved electronically. When the polarization controller 140-2'' is tuned to mode locking status, a stronger component in 20-100 MHz bandis generated. The processing electronics 160 receives the incoming signals tapped from the output coupler 130 to process the analog signal and converts the signals into frequency dependent components. The filtering electronics 165 has different filtering bandelectronics that can extract signals in different frequency ranges, e.g., a DC signal, 20-100 MHz signal that correlates to the mode locking operation, and relaxation signal at around ten kHz. The controller electronics 150 adjusts the polarizationstate based on a control loop algorithm. The polarization state is adjusted by using an adjustment algorithm. The controller electronics 150 applies the adjustment algorithm generated by using the three components from the filtering electronics toachieve a mode locking state. The adjustment algorithm employed by the controller electronics 150 is based on several correlations provided by these signal components. Specifically, the DC signal indicates how the laser operates at the mode lockingstate in the 20-100 MHz. A stronger DC component indicates a weaker mode locking of the laser system. The 20-100 MHz components are directly correlated to the mode locking status. The stronger the signals of the 20-100 MHz components, the better thelaser mode locking performance. The relaxation signal related the stability of this laser operation. It may also use the ratio of them. The controller electronics 150 applies a polarization adjustment algorithm to achieve an ideal operation of modelocking laser system by reducing the DC signal components, increasing the signals of the 20-100 MHz components while stabilizing the relaxation signal. In starting up the laser system, the laser cavity is generally operating at a continuous mode in the beginning with large DC component and a signal component in the 20-100 MHz range is not detected. While tuning the polarization controller, thesignal component at the 20-100 MHz frequency range is gradually increased and the relaxation oscillation is also becoming more stable. The DC component is gradually decreasing. Close to a mode locking condition is achieved. Upon fine-tuning thesystem, a stable and optimal 20-100 MHz and relaxation signal components are measured while a lowest DC component is maintained. A mode locking state is achieved with the tunable driver controlling the polarization controller as described above. Asshown in the drawings above, there are two polarization controllers. There are benefits of controlling and tuning both controllers to further accelerating the progress of the system condition to achieve a mode locking condition. A automatic controlloop can be implemented to fully automate the self-starting process to progressively tuning the system to a mode locking condition without human intervention. Similar to the laser shown in FIGS. 8 and 9, the amplification is achieved by using a short piece of high concentration double cladding Yd-doped fiber (DCYDF) with large mode area (LMA) 105 as shown in FIG. 11. The LMA of the DCYDF combined withshort length help balance the nonlinear effects such as SPM and XPM with the dispersion so the pulse width will not be broadened after amplification. This DCYDF can be a PC fiber as well in balancing the dispersion and SPM. The laser system as shown inFIGS. 8 and 9 in combination with an the gain medium shown in FIG. 11 has the advantages that it is alignment and maintenance free. It is much easier to handle the all-fiber based fiber laser and amplifiers than conventional mode locked solid stateand/or fiber lasers. There are no alignment and realignment issues related. After the fibers and components are spliced together and packaged, there will be no need of specially trained technician for operation and maintenance, which reduce the costand risk significantly in the field applications. Furthermore, it can be easily integrated with other module, such as telescope/focusing system without extra optical alignment effort due to the flexibility of optical fiber. The laser system furthertakes advantage of the fully spectrum of the gain of the YDF and provides a high quality laser that is suitable for processing the nano-material. The laser system is implemented with all photonic crystal fibers for both the gain medium and transmissionfibers in the cavity to compensate both the dispersions and dispersion slope. The photonic crystal (PC) fiber shows novel properties in manipulating its structures such as hollow lattice shapes and filling factors to obtain both normal and anomalousdispersion below 1300 nm range. The PC fiber is used to compensate both dispersions and slope in the cavity and make short pulsed fiber laser by selecting various PC fibers. Further more, due to one of its unique features of smaller effective area thanthe regular single mode fibers, stronger nonlinear effects can be caused in the fiber and its impact on SPM can be utilized to achieve shorter cavity by selecting an appropriate PC fiber. On the other hand, by using the feature of air core PC fiber,larger pulse energy can be extracted. As shown in FIG. 11, a high power amplifier YDF 105 is used to boot the seed pulse inputted from a high power pump 101 through a pump coupling optics 110' to an average power up to 10 W with femetosecond ultra-shortpulse amplification. This is different from the CW (continuous wave) and nano-second (NS) pulse. Special consideration must be taken into accounts of the effects of SPM, XPM, and FWM. The dispersion has to be carefully selected to make all effectsmatched and balanced to avoid any pulse broadening and distortion in the non-linear short pulse fiber transmission modes. Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt becomeapparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention. |
